An important remark is in order. The phase transitions occurring between MeV and GeV, such as those considered here, are responsible for producing a second SGWB. The latter is generically described by a broken power-law peaking around the scales probed by PTA and THEIA, and the location and amplitude of the peak depend on the specific phase transition considered [1]}. Therefore, the total SGWB is the superposition of such a broken power law and the one produced by PBH binary mergers presented above. Although it is out of the scope of this work, it would be interesting to study the properties of two backgrounds, such as their spectral shape and duty cycle [2]}, to see how they can be separated. Their joint detection would provide stronger support to our model.
